Ted:

You've gotten a good answer from Tim about the proper fuse to use, but
you asked another good question that went unanswered and that is all
too often overlooked. That is, what does the fuse protect? The
answer is, as you suggested, the wiring. AC43.13 has wiring size
tables for common A/C wires and the size fuse needed to protect them.
It is a table worth having available somewhere in your shop. By
referring to that table you can know what size fuse will safely
protect the wire (gauge and run) it is connected to. The other
question you asked -- does the fuse protect the instrument -- is more
problematic. Generally speaking it does not because the instrument
has an internal fuse to protect it. Not all do, of course, and some
fuses are so difficult to access and replace that you'll wonder if
you're better off putting a low value fuse before the instrument.
These are individual decisions based on the instrument you're
installing. Fred

Fred LaSor
SoaringNV
Minden, NV